Evaporator for low-pressure refrigerants



Oct. 1, 1946.- J G RE'.D,J 2,408,480

EVAPORATOR FOR LOW PRESSURE REFRIGERANTS Filed Jan. 3., 1945' ATTORNEY Patented Oct. 1, 1946 EVAPORATOR FOR- LOW-PRESSURE REFRIGERANTS v John G. Reid, Jr., Evansville, Ind., ,assignor to I Servel, Inc., New York, Y., a oorporation of Delaware Application January 3, 1945, Serial No. 571,167 I i The present invention relates to refrigeration and more particularly to a refrigeration system including a novel evaporator construction to adapt the system for air conditioning.

While th resent invention may be used with other absorption refrigeration systems, it is particularly adapted for us in a system of the type illustrated and described in the application for United States Letters Patent of Albert R. Thomas, Serial No, 560,214, filed October 25, 1944, entitled Refrigeration. The refrigeration system illustrated and described in the Thomas application operate in a partial vacuum and utilizes water as a refrigerant and a saline solution as an absorbent. The refrigerant and absorbent; are introduced into the system as a water solution of a salt such as lithium chloride, lithium bromide, or the like and the absorbent occurs in the system as a dilute or concentrated water solution of the salt. 7

One of the objects of the present invention is to provide an evaporator or cooling element having vertical tubes extending between upper and lower headers and so constructed and arranged as to distribute liquid refrigerant equally to the plurality of tubes.

Another object is to provide an evaporator of the type indicated in which the liquid refrigerant is supplied to the open upper end of each tube in a thin sheet or film around its entire periphery for gravity flow therethrough.

Another object is to provide an evaporator having vertical tubes of the type indicated with serrations on the-inner periphery of each tube to impede the flow of liquid refrigerant and maintain a thin film of the liquid refrigerant throughout its length.

Another object is to provide an evaporator of the type indicated having a pump circuit for recirculating the refrigerant from the lower header to the upper header.

Still another object is to provide an evaporator of the type indicated which is adapted-to accommodate the large volumes of refrigerant vapor formed in a vacuum system.

These and other objects will become more'apparent from the following description and drawing in which like reference charactersdenote like parts throughout the several views. It is to be expressly understood, however, that the drawing is for th purpose of illustration only and not a definition ofthe limits of the invention, reference being had for this purpose to the appended claims.

In the drawing:

Claims. (o1.62 '119) i Fig. 1 is a diagrammatic view of an absorption refrigeration system incorporating the evaporator of the presentinvention'; and g Fig. 2 is a longitudinal sectional view of the up per portion of an evaporator tubeshowing the internal helical groove forming a plurality of serations on the inner periphery of the tube andja wick to facilitate the initial'wetting of the inner surface of the tube. v i

In the drawing, the invention 'isshown applied to a refrigeration system'of the type illustrated in the Thomas application referred to above. In a refrigeration system of thi type a liquid refrigerant-such as, for example, water is introduced into the evaporator [0, later 'to be described in detail, from a condenser ll through a path of flow including a U-shaped tube l2. Ihe u-shaped tube !2 has. one end connected to a sump l3 at the bottom of the condenser II and its opposit end extends through the top of the evaporator H3. The refrigerant vapor formed in the evaporator it! flows 'to an" absorber [4 where the vapor is absorbed in aliquid absorbent such as, for example, a concentrated water solution of lithium chloride or lithium bromide. The absorber M is in the form of a horizontally arranged cylindrical drum positioned below the evaporator it. A pipe l5 extends upwardly from the top of the absorber l4 and is connected to the emperor-- tor It to provide a path of flow for refrigerant vapor from the evaporator to the absorber.

Absorption liquid enriched with refrigerant or, in other words, a dilute salt solution is conducted from the absorber l 4' to the base ofa generator I 6 in anath of'flow including a conduit l1, pump l8, conduits i9 and 2E], liquid heat'exchanger 2| and conduit 22. The generatorlfi comprises'a plurality of riser tubes 23 enclosed Within an outer shell as to provide a chamber therebetween. Steam is supplied to the chamber inthe generator it from a boiler 25 through a conduit 26. For purposes of illustration a manually operable valve V is shown in the conduit 25 for controlling the amountof steam supplied, to the generator 16 but'it will be understood that a suitable automatically operable valve may be substituted for controlling the amount of steam delivered to the generator it in accordance with thevload. The

boiler 25 isprovided with fire tubes 21 to which heat is supplied from suitable fuel burners 28. An outlet vent 29 is provided adjacent the upper end of the steam chamber in the generator l5 and a condensate return line 30' is provided adjacent the base thereof. The heating of the riser tubes 23 by the Steam causes refrigerant vapor to and conduit l9.

be expelled from the absorption solution and such expelled vapor is effective to raise the absorption solution in the tubes by gas or vapor lift action. The expelled vapor passes from the upper ends of the riser tubes 23 into a vapor separator 3| and thence flows through a conduit 32 to the condenser H where the vapor is liquefied. Liquid refrigerant formed in the condenser ll flows through the U-shaped tube [2 to the evaporator ID, as explained above, to complete the refrigeration cycle.

The raised absorption solution from which refrigerant vapor has been expelled is conducted from the upper portion of the generator IE to the absorber M in a path of flow including a conduit 33, liquid heat exchanger 2| and conduit 34. The end of the conduit 34 extends into the .7

upper portion of the absorber I4 adjacent the inlet from the pipe [5 and has a plurality of apertures or nozzles therein to provide a distributing pipe 35 for dividing the absorption solu tion as it is introduced into the absorber to promote absorption of the refrigerant vapor. The heat liberated by the absorption of refrigerant vapor in absorber I4 is taken up by a cooling medium such as, for example, water which flows upwardly through vertically disposed banks of pipes or coils 36 in the absorber. The cooling water is introduced into the lower ends of the banks of pipes 38 from a supply main 31 and is discharged \from the upper ends of the banks of pipes through a conduit 38. Conduit 38 is connected to the condenser ll so that the cooling water also may be utilized to effect cooling of the condenser. The cooling water is discharged from the condenser ll through a conduit 39.

The system operates in a partial vacuum with the generator [5 and condenser H operating at one pressure and the evaporator IE) and absorber 14 operating at a lower pressure. The pressure differential between the high and low pressure sides of the system is maintained by a liquid column in the up-leg of the U-shaped tube l2 between a condenser l l and evaporator ID. A similar liquid column also is present in the conduit 34 connecting the absorber I 4 and liquid heat exchanger 2!. The pump l8 connected between the absorber M and liquid heat exchanger 2| also maintains the pressure differential between the absorber and generator I 6. The absorption solution in the absorber I4 is circulated continuously through an auxiliary loop circuit to promote absorption of the refrigerant vapor. The auxiliary loop circuit for the absorption solution comprises the conduit l1, pump l8 The upper end of the conduit l1 extends into a sump 40 in the bottom of the absorber l4 and the lower end of the conduit is connected to the inlet of the pump l8. Conduit [9 is connected at one end to the outlet from the pump I8 and its opposite end extends throughout the length of the absorber l4 adjacent the top thereof. A series of nozzles 4| are provided in the end portion of the conduit I9 extending into the absorber M to provide a liquid liquid into a fine spray as it is delivered to the absorber. During operation of the refrigeration system, part Of the circulating absorption solu distributing pipe 42 for dividing the absorption 4 the upper end of the conduit in the separating chamber is protected by a shield 44.

When relatively large refrigeration units utilizing water as a refrigerant are used for air conditioning, a uniform distribution and evaporation of refrigerant in different parts of the evaporator cannot be obtained in the usual types of evaporators due to the low pressure and great volume of refrigerant vapor. A flooded type evaporator cannot be used advantageously in such a refrigeration system because of the hydrostatic head of refrigerant in such an evaporator which raises the boiling point progressively from the top to the bottom of the evaporator and renders the greaterportion of its surface inefiective for the transfer of heat. To avoid such hydrostatic head, it is desirable to maintain a thin film of the liquid refrigerant on the internal surfaces of the evaporator. With relatively small units using water as a refrigerant, it is the practice to flow the refrigerant from the top to the bottom of the evaporator in a shallow stream through a series of substantially horizontal tubes. In such small units a substantial wetting of the interior surfaces of the tubes is obtained and the design can be such that all of the refrigerant will be evaporated during a single pass through the evaporator at full load conditions. With large units, however, due to the increased amount of liquid refrigerant supplied to the evaporator it is practically impossible to maintain a thin film of refrigerant on the interior surfaces of such a series arrangement of horizontal tubes and the liquid refrigerant in the first tubes of the evaporator is apt to be blown out of the tubes by the rapidly expanding refrigerant vapor. Furthermore, it is difiicult to design a horizontal tube evaporator for a large unit which will vaporize all of the refrigerant during a single pass therethrough under varying load conditions.

In accordance with the present invention a novel :form of air conditioning evaporator is provided for use with relatively large refrigeration units. The evaporator Hi comprises a casing 59 forming a central rectangular opening 5| through which the air to be conditioned may be circulated. The casing is of generally hollow construction to provide an upper header 52 above the opening 5|, a lower header 53 below the opening and a relatively large passageway 54 at one side of the opening. The bottom wall of the upper header 52 constitutes a horizontal tube sheet 55 and the top wall of the lower header 53 constitutes a tube sheet 56. The end of the lower header 53 opposite the passageway 54 extends horizontally and vertically to provide a pump chamber 5?. .One end of the upper header 52 converges into a cylindrical pipe section 58 having a flange 59 at its end for connection to the pipe 15.

A plurality of vertical pipes or cooling tubes 60 extend across the opening 5i and through the tube sheets 55 and 58 in the upper and lower headers 52 and 53. .The upper end portions of the tubes 60 extend abovethe tube sheet 55 in the upper header 52 with all of the ends arranged at the same level in a horizontal plane. A wall 6i extends upwardly from the tube sheet 55 and between the side walls of the header 52 to provide a dam for maintaining a pool of liquid refrigerant around the upwardly projecting ends of the tubes Bil. While a single row of the tubes are illustrated in the drawing, it will be understood that a plurality of rows of tubes may be provided and the tubes are provided with 'fins 63 for in of the tubes 60 are roughened or serrated to impede the flow of refrigerant therethrough and maintain a thin film of liquid refrigerant thereon. As, illustrated in Fig. 2, the serrations on the interior of each tube is formed by an internal helical groove 64 in which refrigerant flows from the top to the bottom of the tube by gravity. Due to the size and shape of the helical groove .64 the liquidrefrigerant creeps up along the surface of the tube between adjacent grooves due to capillary action to wet the entire surface of the tube and form afilm ofliquid refrigerant thereon.

While not essential to the operation of the evap- .of the tube. As illustrated in Fig. 2, the starting device is formed by a fine mesh metallic screen looped over the sides of the upwardly projecting end of the tube. In place of a metal screen a fabric sleeve or wicking may be used.

The liquid refrigerant in the lower header 53 is recirculated through the evaporator II! by a pump 65in the pump chamber 51. The pump 55 is positioned below the liquid level in the lower header 53 and the pump'is driven by a shaft 66 extending from an hermetically sealed motor 67 mounted on the evaporator casing. A conduit 63 is connected to the outlet from thepump 65 and extends upwardly and then horizontally throughout substantially the entire length of the upper header 52. The portion of the conduit 68 extending into the header 52 has aseries of outlet apertures or nozzles 69 and constitutes a liquid distributing pipe Hi for supplying the recirculated refrigerant to the upper header.

An overflow conduit 15 is provided in the evaporator IO to prevent the accumulation of an excessive amount of liquid refrigerant therein. The upper end of the conduit l5 extends into the evaporator in to a predetermined height above the bottom thereof and below the lower ends of the tubes 50. The opposite end of the conduit 15 may be connected to any part of the refrigeration in'the helical grooves 64 from the top to thebotsystem and as herein illustrated it is connected Hi to dilute the absorption solution and maintain the concentration below the saturation point at which crystallization and precipitation of salt from the solution will occur. One form of the invention having now been described in detail the mode of operation is explained as follows.

Steam supplied to the generator vaporizes the refrigerant therein which flows upwardly through the tubes 23 and raises the absorption solution by vapor lift action. The refrigerant vapor flows from the separating chamber 3! through a conduit 32 to the condenser H where the vapor is condensed to a liquid. The liouid refrigerant flows from the condenser H through the U-shaped tube !2 into the upper header 52 of the evaporator. Due tothe dam f! in the upper header 52, a pool of the li uid refrigerant accumulates around the end portions of theevaporator tubes projecting above the tube sheet 55. When a suflicient amount of liquid refrigerant has accumulated in the header 52, the liquid level will rise to the horizontal level of the upper ends of the tubes 60 and will flow into the tubes at the .innerperiphery. thereof. Due to the equalheight of the ends of the evaporator tubes 58 above the tube sheet 55 the liquid refrigerant will be distributed equally in all of the tubes Bi] and will be supplied to each tube in a thin film around its entire inner periphery. When a starting device 82, as illustrated in Fig. 2, is provided on the upwardly projecting end of each of the tubes 58, a small amount of the liquid refrigerant is siphoned into the interior of the tube by capillary action to wet the interior before the liquid refrigerant rises to a level above the upper end of the tube. Such a starting device assists in the initial wetting of the tube to insure equal distribution of refrigerant in the plurality of tubes, "The liquid refrigerant will flow downwardly by gravity tom of the tubes 60 and due to the shape and size of the grooves the. liquid refrigerant will creep between adjacent grooves to wet the interior surface of the tubes throughout their length. In other words, the helical serrations 64 on the interior of the evaporator tubes 68 will cause a thin film of the liquid refrigerant to be formed thereon. The liquid refrigerant in excess of that evaporated during a single pass through the evaporator tubes 5i) will flow through the tubes to the lower header 53.

The pump 65 continuously recirculates the refrigerant from the lower header 53 to the upper header '52 and delivers the refrigerant through the nozzles 69 in the form of a spray to promote evaporation of the refrigerant. Due to the relatively thin film of liquid refrigerant on the internal surfaces of the tubes 60 the evaporation of the refrigerant occurs at the surface of the liquid so that liquid refrigerant will not be blown from the tubes. Furthermore the recirculation of the refrigerant insures complete and equal evaporation throughout all arts of the evaporator.

The evaporation of the refrigerant in the evaporator lfl will reduce the temperature of the refri erant to a value corresponding to the pressure prevailing in the evaporator. Thus the evaporation of the refrigerant acts to cool the body of refrigerant supplied to the header 52 from the condenser I l and recirculated by the pump 65. The air to be conditioned flows through the opening 5| and around the tubes 59 in the evaporator H! and is chilled bv its contact with the tubes; the air transferring its heat through the tubes to evaporate more of the refrigerant'to maintain a temperature correspondins: to the pressure in the] evaporator.

The refri erant vapor flows upwardly through the tubes 6!! tothe upper header nd from the lower header 53 through the relativelylarge passage 54 to the upper header 52 and thence flows through the pipe to the absorber M where it is absorbed in the absorption solution. The absorption solution in the upper part of the generator 16 flows through the conduit 33, liquid heat exchanger 2! and conduit 34 to the liquid distributing pine 35 where it is delivered into the absorber. The absorption solution in the absorb or M is continuousl recirculated through the conduit-i7. pump it! and conduit 89 to the li uid distributing pipe 42 in the absorber which delivers the absorption solution in a fine spray to promote absorption. A portion of the recirculated absorption solution is continuously diverted through the conduit 2!], liquid heat exchanger 2| and conduit22 back to the base of the generator.

If for any reason the liquid refrigerant tends to accumulate in the evaporator Hi, the overflow conduit 15 will drain the refrigerant to the conduit 22 adjacent the base of the generator to maintain the concentration of the absorption solution within permissible limits.

It will now be observed from the foregoing specification that the present invention provides an evaporator of novel construction including a plurality of vertical tubes extending between upper and lower headers. It will further be observed that the evaporator of the present invention is so constructed and arranged as to maintain a film of liquid refrigerant in all parts of the evaporator and is adapted to accommodate the large volumes of refrigerant vapor which are formed in a vacuum system. It will still further be observed that the present invention provides for continuously circulating refrigerant from the lower header to the upper header to provide an ample supply of liquid refrigerant to insure the wetting of the internal surfaces of all of the evaporator tubes.

While a preferred refrigeration system and evaporator construction are herein illustrated and described it is to be understood that other refrigeration systems may be used and that modifications may be made in the construction and arrangement of the elements in the evaporator without departing from the spirit or scope of the invention. Therefore without limiting myself in this respect reference is to be had to the appended claims for a definition of the limits of the supplying liquid refrigerant to the upper header which overflows into the end portions of the tubes projecting above the tube sheet, said tubes having internalserrations to retard the flow of refrigerant through the tubes to maintain a film of liquid refrigerant on the interior surfaces thereof, and means for recirculating liquid refrigerant from the lower to the upper header.

2. In an absorption refrigeration system, a generator, a condenser, an evaporator, an absorber, conduit means interconnecting the elements to provide circuits for a refrigerant and absorbent, said system operating in a partial vacuum and utilizing water as a refrigerant and a salt solution as an absorbent, said evaporator comprising a casing forming a central opening with a header above the opening, a header below the opening and a relatively large passageway between the upper and lower headers at one side of the opening, a plurality of vertical tubes extending between the upper and lower headers, means for supplying liquid refrigerant to the upper header to cause it to flow by gravity through the plurality of tubes, said interconnecting means including a conduit connecting the upper header to the absorber, and a conduit having one end extending a predetermined height above the bottom of the lower header and connected to another iii) part of the system to control the amount of refrigerant in the evaporator by draining excess refrigerant to the other part of the system.

3. An evaporator for use in a refrigeration system comprising a casing forming a central opening with a header above the opening, a header below the opening and a relatively large passageway between the upper and lower headers at one side of the opening, said upper header having an inlet for liquid refrigerant and an outlet for refrigerant vapor, the bottom wall of the upper header forming a horizontal tube sheet, a plurality of vertical tubes extending between the upper and lower headers with the ends of the tubes projecting above the tube sheet in the upper header, structure in the upper header for retaining a pool of liquid refrigerant which overflows into the upwardly projecting ends of the tubes, a pump in the lower header, and a conduit connecting the pump and the upper header to recirculate refrigerant from the lower header to the upper header.

4. An evaporator for use in a refrigeration system comprising a casing forming a central opening with a header above the opening, a header below the opening and a relatively large passageway between the upper and lower headers at one side of the opening, said upper header having an inlet for liquid refrigerant and an outlet for refrigerant vapor, the bottom wall of the upper header forming a horizontal tube sheet, a plurality of vertical tubes extending between the upper and lower headers with the ends of the tubes projecting above the tube sheet in the upper header, a dam in the upper header for retaining a pool of liquid refrigerant which overflows into the upwardly projecting ends of the tubes, said tubes having internal serrations to impede the flow of refrigerant therethrough and cause a wetting of the internal surface of the tubes, a pump in the lower header, and a conduit connecting the outlet from the pump to the upper header for recirculating refrigerant from the lower header to the upper header.

5. A unitary evaporator comprising a casing forming a central opening with a header above the opening, a header below the opening and a relatively large passageway between the upper and lower headers at one side of the opening, said upper header having an inlet for liquid refrigerant and an outlet for refrigerant vapor, the bottom wall of the upper header forming a horizontal tube sheet, a plurality of vertical tubes extending between the upper and lower headers, a dam in the upper header for retaining a pool of liquid refrigerant which overflows into the ends of the tubes, the end of the lower header opposite the passageway being extended horizontally and vertically at the side of the opening to provide a pump chamber, a pump in the pump chamber, an hermetically sealed motor mounted on the casing and connected to drive the pump, and a conduit connected to the outlet from the pump and extended into the upper header, said pump and conduit recirculating refrigerant from the lower to the upper header.

JOHN G. REID, JR. 

